Cell Membrane Study Guide

Transcription

1 Cell Membrane Study Guide U1.3.1: Phospholipids form bilayers in water due to the amphipathic properties of phospholipid molecules (Oxford Biology Course Companion page 26). 1. Explain why phospholipids form bilayers in water, with reference to the amphipathic properties of phospholipids. The hydrophilic heads are attracted to water while the hydrophobic tails are repelled by water. The bilayer alignment allows the tails to be removed from water while the heads are exposed to extracellular fluid/cytosol NOS 1.3.1: Using models as representations of the real world-there are alternative models of membrane structures (Oxford Biology Course Companion page 26). 2. Describe the observations and conclusions drawn by Gorter and Grendel in discovering the structure of cell membranes. Gorter and Grendel (1925) investigated the surface area of membranes. They noticed that the surface area of an intact red blood cell was half of the surface area of the lipids when spread on a surface. They concluded that the cell membrane is made of a bilayer of lipids with head groups facing the inside and outside of the cell and the tails of each layer facing inward towards each other. S1.3.2: Analysis of evidence from electron microscopy that led to the proposal of the Davson- Danielli model (Oxford Biology Course Companion page 28).

2 3. Describe the observations and conclusions drawn by Davson and Danielli in discovering the structure of cell membranes. In 1935 Davson and Danielli proposed the protein-lipid sandwich model of the cell membrane. In electron micrographs, they observed two dark parallel lines with a light region in between. Since proteins appear dark and lipids appear light in micrographs, Davson and Danielli proposed that the phospholipid bilayer was embedded between two layers of proteins. S1.3.3: Analysis of the falsification of the Davson-Danielli model that led to the Singer- Nicolson model (Oxford Biology Course Companion page 29). 4. Compare the Davson-Danielli model of membrane structure with the Singer-Nicolson model.

3 Singer and Nicolson proposed a membrane model that incorporated evidence about membrane proteins that did not comply with the Davson-Danielle model. Rather than having proteins on the surface of the phospholipids, Singer-Nicolson proposed a model in which proteins were embedded within and through the membranes, called the Fluid-Mosaic Model. NOS1.3.2: Falsification of theories with one theory being superseded by another-evidence falsified the Davson-Danielli model (Oxford Biology Course Companion page 27). 5. Describe why the understanding of cell membrane structure has changed over time. As tools and technologies advance, our understanding of biological structures and functions also improves. Techniques such as freezefracture, cell fusion, fluorescent antibody tagging and protein extraction enabled scientists to gain a more accurate understanding of the structure of cell membrane proteins. S1.3.1: Drawing of the fluid mosaic model (Oxford Biology Course Companion page 31). 6. Draw and label the structure of membranes. Include: - Phospholipid bilayer - Integral proteins shown spanning the membrane - Peripheral proteins on membrane surface - Protein channels with a pore - Glycoproteins with a carbohydrate side chain - Cholesterol between phospholipids in the hydrophobic region - An indication of thickness ( 10nm) U1.3.2: Membrane proteins are diverse in terms of structure, position in the membranes and function (Oxford Biology Course Companion page 30).

4 7. State the primary function of the cell membrane. The cell membrane is semi-permeable and controls the movement of substances in and out of cells. 8. List at least four functions (with example) of membrane bound proteins. 1. Receptor proteins communicate signals between the cells internal and external environments (i.e. hormone receptors) 2. Transport proteins move ions and molecules across the membrane (i.e. aquaporin transports water) 3. Enzymes catalyze reactions (i.e. ATP synthase) 4. Adhesion molecules anchor the cell to other cells (i.e. adherin) 5. Recognition proteins identify the cell type (i.e. major histocompatibility complex proteins) 9. Contrast the two types of transport proteins: pumps and channels. Channel proteins are used for passive transport of molecules, often shaped like pores/tunnels. Move from high to low concentrations. Pump proteins are used for active transport of molecules. Move from low to high concentrations. U1.3.3: Cholesterol is a component of animal cell membranes (Oxford Biology Course Companion page 32). A1.3.1: Cholesterol in mammalian membranes reduces membrane fluidity and permeability to some solutes (Oxford Biology Course Companion page 33). 10. Describe the structural placement AND function of cholesterol within the cell membrane.

5 Cholesterol fits between phospholipids in the cell membrane with its hydroxyl (OH) group by the heads and the hydrophobic rings by the fatty acid tails. Cholesterol acts as a regulator of membrane fluidity (which is the viscosity of the cell membrane). At high temperatures, it stabilizes the membrane and raises the melting point. At low temperatures, it prevents phospholipids from packing too close together which would lead to stiffening. The membrane fluidity effects how permeable the structure is to solutes Too fluid too much permeability Too stiff not enough permeability U1.1.3: Cell Surface to volume is an important limitation to cell size (Oxford Biology Course Companion page 9). 11. Outline the activities occurring in the volume and at the surface of the cell. Volume: metabolic reactions occur in the cytoplasm which require nutrients and produce waste.

6 Surface:comprised of cell membrane through which nutrients and gases move into the cell and metabolic wastes exit. 12. Explain why cells are often limited in size by the SA:V ratio. If cell size increases, the surface area:volume ratio decreases. This means that with larger cells, there is less surface area relative to the amount of volume so materials may not move as efficiently as needed. 13. List three adaptations of cells that maximize the SA: volume ratio. Cell Division to maintain size, Cell Compartmentalization, Folding of plasma membrane on surface of cell (villi) U1.4.1: Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport (Oxford Biology Course Companion page 35). 14. Describe simple diffusion using two examples simple diffusion of molecules into and out of cells. Net movement of molecules from areas of higher concentration to lower concentration without the input of energy. Example: Alveoli cells and Cornea 15. Outline factors that regulate the rate of diffusion. Concentration of molecules, Temperature, and Pressure (all are direct relationships) 16. Describe one example of facilitated diffusion through a protein channel. Movement of molecules from higher to lower concentration through a channel protein without the use of energy. CFTR channel moves chloride ions from higher concentration inside the cell to areas of lower concentration outside the cell. 17. Define osmosis and predict the direction of water movement based upon differences in solute concentration. The movement of water by diffusion across a membrane. Water moves from hypotonic solutions into hypertonic solutions (moves toward the higher solute concentrations) 18. Compare active transport and passive transport with examples for each Active Transport: requires energy input, moves against the concentration gradient. EX: Protein pump

7 Passive Transport: does not require energy input, moves with the concentration gradient. EX: Aquapore A1.4.2: Tissues or organs to be used in medical procedures must be bathed in a solution with the same osmolarity as the cytoplasm to prevent osmosis (Oxford Biology Course Companion page 44). 19. Explain what happens to cells when placed in solutions of the same osmolarity, higher osmolarity and lower osmolarity. S1.4.1: Estimation of osmolarity in tissues by bathing samples in hypotonic and hypertonic solutions (Oxford Biology Course Companion page 41). 20. Determine osmolarity of a sample given changes in mass when placed in solutions of various tonicities. The osmolarity of a sample is the position at which the % change in mass is 0.

8 U1.4.2: The fluidity of membranes allows materials to be taken into cells by endocytosis or released by exocytosis (Oxford Biology Course Companion page 34/35). 21. Describe the fluid properties of the cell membrane and vesicles. Fluidity: viscosity flow of phospholipids in cell membrane and organelles of the endomembrane system (including vesicles). Fluidity is impacted by: temperature, fatty acid length, fatty acid fluidity, presence of chloresterol 22. Explain vesicle formation via endocytosis and give 2 examples of materials brought into the cell via endocytosis. Cells actively transport molecules into the cell. -Engulfs molecules into a vesicle form through the cell membrane capture and fusion. it includes the cell membrane Examples: 1) White blood cells eating bacteria to kill it. 2) Amoeba eating bacteria as a food source. 23. Explain release of materials from cells via exocytosis and give 2 examples of materials released from a cell via exocytosis. -Secretory vesicles. -Wastes moves towards the plasma membrane. Fuses with the membrane, releasing contents to extracellular space. Examples: -Secretion of neurotransmitters at synaptic terminus. -Secretion of digestive juices from exocrine cells.